When I change the language to zh-CN which made by myself,the command of "about: plugins" couldn't show the all installed plugins.But if the language is en-US,that works fine.
why? Do the defects of translation cause this?

Have a look to this part of translation. As you used an older translation as base, try just for that part to restore english strings to see if it changes something.

Have you some error message when starting or using K-Meleon?
Does the plugins that are missing work correctly ?
It seems that the problem occurs with Flashplayer. Have you tried to rename it (npswf32.lld for example) to see if it occurs also with the next plugin ?

You could perhaps rename all the missing plugins in that way and unmake such rename one each after the other, to see what happens...

Thanks for your help!
No error message when starting or using K-Meleon,and all the plugins that are missing work correctly.
I had renamed the NPSWF32.dll as npswf32.dll,but even though I keep it as NPSWF32.dll,the problem still occurs.

Finally,I keep some sentences of the plugins.properties in english,and the problem doesn't occur again,all the installed plugins could be shown now.